Oximetry has been used for non-invasive monitoring of blood and tissue oxygen saturation levels using devices such as finger tip pulse oximeters. Pulse oximeters determine a patient's arterial blood oxygen saturation levels, which is particularly useful in monitoring patients while under anesthesia. Typically, a finger tip oximeter transmits a light from an LED through the finger tip to a photoreceptor opposite the LED which detects the absorption of predetermined wavelengths of light. Because the light absorption for oxygenated and reduced hemoglobin are different, the absorption level may be used to determine oxygen saturation levels. Light reflectance has also been used to determine oxygen saturation levels. The term light includes the entire electromagnetic radiation range and specifically includes the ranges of light used to determine blood oxygen concentration. Various methods of measuring oxygen saturation levels using pulse oximetry are known in the art. Some of these methods are described, for example in U.S. Pat. Nos. 4,759,369 and 4,807,631.
Oximetry is based on the principle that the color of blood is a function of saturation of hemoglobin with oxygen. The absorption or reflectance of light is different for oxygen saturated hemoglobin (oxyhemoglobin) and reduced hemoglobin. The absorption or reflectance also varies for each depending on the wavelengths of light directed toward the blood or tissue. The differences in light absorption (measured as light transmission or reflection) between reduced and oxyhemoglobin as related to wavelength can be described by the molecular extinction coefficients of hemoglobin for each wavelength. Using the hemoglobin extinction curves based on the absorption or conversely reflectance of light directed toward vascularized tissue or organs, oxygen saturation can be calculated from a ratio derived from an absorption formula known as Beer's Law. These relationships are well known in the art and are routinely used in one form or another to determine oxygen saturation levels in blood or tissue.
Typically, two or more wavelengths of light are used to illuminate tissue. The degree of absorption of light is determined by either measuring the amount of light transmitted through the tissue or the amount of light backscattered from the tissue. The term backscattered is meant herein to be diffuse as opposed to specular reflection. The amount of light reflected is measured using photodiodes which convert the light to a corresponding signal. The signal is normalized and processed using known signal processing techniques based on Beer's Law, to eliminate variables in the signal due to varying skin pigmentation, thickness of skin, perfusion, patient motion, etc. Thus, for example, in pulse oximetry, the normalized signal represents the pulsed waveform caused by the pulsing of the arterial blood. The pulsed signal is used in calculating arterial oxygen saturation. Because the detected pulsatile waveform is produced solely from arterial blood, using the amplitude at each wavelength and Beer's law allows exact beat to beat continuous calculation of arterial hemoglobin oxygen saturation with minimal interference from surrounding venous blood, skin, connective tissue or bone. The resulting information is used to calculate arterial blood oxygen saturation.
Oximetry has also been described for determining either organ or tissue oxygen consumption. In one method, the metabolic rate of an internal body organ or tissue is determined by blocking the blood supply to the organ or tissue. Using similar light reflectance or absorbance techniques, oxygen saturation levels are measured over a period of time and are used to determine tissue oxygen consumption as a function of time. An example of such methods and devices may be found in U.S. Pat. Nos. 4,463,762 and 4,513,751.